In recent years, devices that treat image information as digital and transmit or store the information with high efficiency, or for example devices complying with systems of MPEG and the like in which compression is performed by an orthogonal transform such as a discrete cosine transform and motion compensation, have been spreading in broadcasting stations and ordinary households.
MPEG2 (ISO/IEC 13818-2), in particular, is defined as a general-purpose image coding system, and is now used widely in a wide range of applications for professional use and consumer use. The use of this MPEG2 compression system can achieve excellent image quality by assigning an amount of code (bit rate) of four to eight Mbps in a case of an interlaced image of a standard resolution with 720×480 pixels, for example. Excellent image quality can also be achieved by assigning an amount of code (bit rate) of 18 to 22 Mbps in a case of an interlaced image of a high resolution with 1920×1088 pixels.
MPEG2 was intended for high-image-quality coding to be adapted mainly for broadcasting, and did not make provisions for a coding system with a lower amount of code (bit rate), that is, a higher compression ratio than MPEG1. With the spread of portable terminals, a need for such a coding system is expected to increase in the future. Accordingly, an MPEG4 coding system was standardized. A standard for an image coding system was approved as an international standard of ISO/IEC 14496-2 in December 1998.
Further, standardization of a standard referred to as H.26L (ITU-T Q6/16 VCEG) has recently been under way with an original objective of image coding for videoconferences. It is known that H.26L requires a larger amount of operation for coding and decoding but achieves a higher coding efficiency compared to conventional coding systems such as MPEG2, MPEG4, and the like. In addition, as part of activities of MPEG4, standardization for achieving higher coding efficiency on the basis of this H.26L is now under way as Joint Model of Enhanced-Compression Video Coding. As for a standardization schedule, an international standard was established under a name of H.264 and MPEG-4 Part 10 (Advanced Video Coding, hereinafter written as “H.264/AVC”) in March 2003.
Further, as an extension thereof, standardization of FRExt (Fidelity Range Extension) including coding tools necessary for business use such as RGB, 4:2:2 and 4:4:4 as well as an 8×8 DCT and a quantization matrix defined in MPEG2 was completed in February 2005. Thereby, the H.264/AVC system is used as a coding system capable of excellently representing even film noise included in movies, and is used in a wide range of applications such as Blu-Ray (registered trademark).
In such a coding and decoding process, image data is coded in block units. In addition, in decoding coded data, as shown in Patent Document 1, for example, block distortion is suppressed by performing filtering on the basis of a block boundary strength and a quantization parameter.
Further, there have recently been increasing needs for coding with a still higher compression ratio, such as a desire to compress images of about 4000×2000 pixels or a desire to distribute high-definition images in an environment with a limited transmission capacity such as the Internet. Thus, as in Non-Patent Document 1, setting a macroblock size to a size larger than that of MPEG2 or H.264/AVC, for example a size of 32 pixels×32 pixels, is proposed. Specifically, in Non-Patent Document 1, a hierarchical structure is adopted for macroblocks, whereby compatibility with macroblocks in H.264/AVC is maintained for 16×16 pixel blocks and smaller blocks, and larger blocks are defined as a superset thereof.